1. Field
The present disclosure relates to hyper-branched polymers, electrodes and electrolyte membranes for fuel cells, each of which includes the hyper-branched polymers, and fuel cells including at least one of the electrodes and the electrolyte membranes.
2. Description of the Related Art
Fuel cells that include a polymer electrolyte membrane as an electrolyte operate at relatively low temperatures and may be manufactured in a small size. Thus, such fuel cells are expected to be used as energy sources in electric vehicles and in distributed generation systems. Perfluorocarbon sulfonic acid-based polymer membranes, such as NAFION (registered trademark), are commonly used as polymer electrolyte membranes for fuel cells. However, in order for such polymer electrolyte membranes to efficiently realize proton conductivity, moisture is needed and thus the polymer electrolyte membranes need to be humidified. In addition, in order to enhance cell system efficiencies, polymer electrolyte membranes need to be operated at a high temperature, for example, at least 100° C. However, the moisture in the polymer electrolyte membrane is evaporated and depleted at such a temperature, which reduces the effectiveness thereof.
To address such problems, non-humidified electrolyte membranes, which may operate at temperatures of at least 100° C., without humidification, have been developed. For example, polybenzimidazole doped with a phosphoric acid is disclosed as a material for forming a non-humidified electrolyte membrane.
In low temperature perfluorocarbonsulfonate polymer electrolyte membrane fuel cells, in order to prevent defective gas diffusion in an electrode, in particular, a cathode, which may be caused by water (product water) generated during electric power production in the electrode, hydrophobic electrodes including polytetrafluoroethylene (PTFE) have been used.
In addition, phosphoric acid fuel cells, which operate at temperatures of about 150 to about 200° C., include a liquid phosphoric acid as an electrolyte. However, a large amount of the liquid phosphoric acid is included in electrodes and thus interferes with gas diffusion in the electrodes. Therefore, an electrode catalyst layer in which a polytetrafluoroethylene (PTFE) waterproofing agent is added to an electrode catalyst and which prevents fine pores in the electrodes from being clogged by the phosphoric acid, has been used.
In addition, in fuel cells including a polybenzimidazole (PBI) electrolyte membrane, which uses a phosphoric acid as a high-temperature and non-humidified electrolyte, in order to improve surface contact between electrodes and the electrolyte membrane, a method of impregnating the electrodes with a liquid phosphoric acid and a method of increasing a loading amount of metal catalysts have been used. However, such fuel cells do not produce improved properties.
When air is supplied to a cathode, activation takes about a week even if an electrode composition is optimized in a solid polymer electrolyte membrane doped with a phosphoric acid. Although performance of fuel cells may be improved and activation time may be reduced by replacing air with oxygen, use of oxygen is not proper in terms of commercialization. In addition, a homogeneous polymer electrolyte membrane using the PBI is not satisfactory in terms of mechanical characteristics, chemical stability, or capability of retaining a phosphoric acid at high temperature. Thus, there is a demand for further improvement.